General Impact
C. fornicata has been reported to alter sediment characteristics (by removing a huge volume of suspended organic material from the water column, and depositing that filtered material on the bottom as pseudofeces). It is also reported to decrease the abundance of certain suprabenthic species (such as mysids) (Vallet et al. 2003). Other studies (de Montaudouin et al. 1999), show that the presence of C. fornicata does not affect the benthic community and spatial competition with other macrozoobenthic species didn't occur, but that the habitat became more heterogeneous. JNCC (2002) states that, "C. fornicata competes with other filter-feeding invertebrates for food and space. It is considered a pest on commercial oyster beds, competing for space and food, while depositing mud on them and the mud rendering the substratum unsuitable for the settlement of spat." Some experimental studies de Montaudouin et al. (1999) conclude that the potential competition of C. fornicata with oysters (Ostrea edulis), populations did not show much overlap, and that C. fornicata provided the required niches for further hard-substrata species and that a rich association could be built on the initial basis of Crepidula alone that the competition of C. fornicata on oyster growth was negligible compared with the effect of competition by oysters themselves (intraspecific competition). Grall and Hall-Spencer (2003) state that C. fornicata is one of many reasons for the decline in local maerl bed habitats in Britain. Live maerl thalli become covered in Crepidula and the interstices of the deposit become clogged with silt; this kills the maerl thalli and dramatically alters associated maerl communities.
The other major reason for the decline in local maerl bed habitats in Britain being industrial exploitation, first by sucking and dredging tons of living material and secondly by depositing overboard tons of suspension matter on or near the beds (Blanchard, M., pers. comm., 2005).
Le Pape et al. 2004 showed the negative effect of this invasive species on the density of young-of-the-year sole Solea solea in coastal nursery areas of the Bay of Biscay (France). Other impacts include increase in the levels of sediments; and, when limpet densities raise, the volumn of shell-attached fauna raises and endogean (domain immediately beneath the ground surface) fauna disappear regularly (Blanchard, M., pers. comm., 2005).
Location Specific Impacts:North Sea (Atlantic Ocean) 
Predation: Predate upon native mussel beds. France
Habitat alteration: Vallet et al (2003) state that, "By removing a huge volume of suspended organic material from the water column, C. fornicata deposits that filtered material on the bottom as pseudofeces and thus alters sediment characteristics." Bay of Brest (France)
Modification of natural benthic communities: Chauvaud et al (2000) conclude that the extensive spreading of Crepidula fornicata has modified the trophic structure of benthic communities by increasing suspension-feeder biomass. The following hypotheses on ecosystem functioning have been made: (1) the decrease of chlorophyll biomass during the first spring bloom results from silicic acid limitation and increased suspension feeder activity, (2) benthic filtration and biodeposition activities enhance biogenic silica retention at the sediment-water interface, and (3) recycling of trapped biogenic silica maintains diatom populations by providing silicic acid in summer and reduces primary production seasonality. These hypotheses suggest that benthic organisms control the export rate of biogenic silica towards the open-water ocean and thus the specific composition of secondary phytoplankton blooms in the Bay. Bay of Marennes-Oleron (France)
Competition: Bacher (1991, in de Montaudouin et al 1999) demonstrated that the competition of Crepidula fornicata on oyster growth was negligible compared with the effect of competition by oysters themselves (intraspecific competition), in the Bay of Marennes-Oléron (first oyster harvest in France).
C. fornicata competes with cultured and fished bivalves (Dupont and Viard, 2003).
Modification of natural benthic communities: Montaudouin and Sauriau, (2000) conclude that the invasion by Crepidula fornicata in the Marennes-Oleron Bay (French Atlantic coast) since the 1970's may have altered subtidal habitats and facilitated the recruitment and spread of some annelid polychaetes which were not previously reported there. Bay of Saint-Brieuc (France)
Modification of natural benthic communities: Vallet et al., 2001 states that, "In the Bay of Saint-Brieuc, the presence of Crepidula fornicata could have a major effect on the suprabenthic community by increasing species number and diversity. This suggests that, for slow swimmers such as decapods, dead and live individual shells of C. fornicata provide new habitats where they can hide." Granville (France)
Habitat alteration: Ehrold et al (1998) report that, "A comparison between a surperficial sediment map of the bay made by Caine and ourselves, in 1996, shows that several muddy areas have appeared during the last twenty years, in particular a large one, in front of Granville where the mud content reaches 50% and where the slipper-limpet population covers more than 50% of the seafloor." Mont-Saint-Michel Bay (France)
Habitat alteration: Ehrold et al (1998) report that since 1980, several mudflat sediments have rapidly formed in Mont-Saint-Michel Bay. The appearance of these deposits in areas unfavorable for the decantation and stability of mud sediments coincides with the proliferation of a gastropod mollusc (C. fornicata). The piling and the agglomeration of individuals favours the retention of these bio-agglomerates and the permanent installation of biogenic muds and modifies radically and irreversibly the nature and the texture of sediment to the specific advantage of this species. Ehrold et al (1998) report that, "A comparison between a surperficial sediment map of the bay made by Caine and ourselves, in 1996, shows that several muddy areas have appeared during the last twenty years, in particular a large one, in front of Granville where the mud content reaches 50% and where the slipper-limpet population covers more than 50% of the seafloor." Norman Gulf (France)
Economic/Livelihoods: Crepidula fornicata has become a serious pest on oyster beds and has caused many traditional oyster fisheries to be abandoned (e.g. in the Norman Gulf, France) (Blanchard, 1997). Jersey
Competition: Crepidula fornicata is undoubtedly displacing native creature (Syvret, 2004 in Varnham, 2006).
Fouling: Crepidula fornicata also creates surface fouling problems (Syvret, 2004 in Varnham, 2006). Sweden
Modification of natural benthic communities: Wallentinus and Jansson, (1999) identify Crepidula fornicata as one of the intruduced fouling organisms whaich have practical and ecological effects on the Swedish west coast. United Kingdom (UK)
Modification of natural benthic communities: Grall and Hall-Spencer (2003) state that Crepidula fornicata is one of many reasons for the decline in local maerl bed habitats in Britain. Live maerl thalli become covered in Crepidula and the interstices of the deposit become clogged with silt; this kills the maerl thalli and dramatically alters associated maerl communities." Poole Harbour (United Kingdom (UK))
Competition: Occupies the same niche as native oysters. Displaces natives (Dyrynda, 2001).
Modification of natural benthic communities: Grall and Hall-Spencer (2003) state that Crepidula fornicata is one of many reasons for the decline in local maerl bed habitats in Britain. Live maerl thalli become covered in Crepidula and the interstices of the deposit become clogged with silt; this kills the maerl thalli and dramatically alters associated maerl communities." Helford River (United Kingdom (UK))
Modification of natural benthic communities: Grall and Hall-Spencer (2003) state that Crepidula fornicata is one of many reasons for the decline in local maerl bed habitats in Britain. Live maerl thalli become covered in Crepidula and the interstices of the deposit become clogged with silt; this kills the maerl thalli and dramatically alters associated maerl communities."
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